1. Field of the Invention
The present invention relates to a resist composition used for microfabrication in manufacturing process of a semiconductor device, for example, for a lithography using an ArF excimer laser of a 193 nm wavelength as a light source, especially for an immersion photolithography in which water is inserted between a projection lens and a wafer, and to a resist-patterning process using the same.
2. Description of the Related Art
As LSI is progressing toward a higher integration and a faster speed in recent years, further miniaturization of a pattern rule is required. Under such a movement, a lithography using a light exposure, which is a widely used technology today, is reaching a limit of its resolution power inherent to a wavelength of a light source.
Heretofore, light-exposure using a light source of a g-line (436 nm) or an i-line (365 nm) of a mercury lamp as an exposure light was broadly adopted in forming a resist pattern. As a mean for further miniaturization, shifting to a shorter wavelength of an exposing light was assumed to be effective. As a result, in a mass production process after DRAM (Dynamic Random Access Memory) with 64-megabits (0.25 μm or less of a processing dimension), a KrF excimer laser (248 nm), a shorter wavelength than an i-line (365 nm), was used in place of an i-line as an exposure light source.
However, in production of DRAM with an integration of 256 M, 1 G and higher which require further miniaturized process technologies (process dimension of 0.2 μm or less), a light source with a further short wavelength is required, and thus a photo lithography using an ArF excimer laser (193 nm) has been investigated seriously since about a decade ago.
At first, an ArF lithography was planned to be applied to a device-manufacturing starting from a 180-nm node device, but a KrF excimer laser lithography lived long to a mass production of a 130-nm node device, and thus a full-fledged application of an ArF lithography will start from a 90-nm node. Further, a study of a 65-nm node device by combining with a lens having an increased NA till 0.9 is now underway.
Further shortening of wavelength of an exposure light is progressing towards the next 45-nm node device, and for that an F2 lithography with a 157-nm wavelength became a candidate. However, there are many problems in an F2 lithography: an increase in cost of a scanner due to the use of a large quantity of expensive CaF2 single crystals for a projector lens; extremely poor sustainability of a soft pellicle, which leads to a change of an optical system due to introduction of a hard pellicle; a decrease in an etching resistance of a resist film; and the like. Because of these problems, it was proposed to postpone an F2 lithography and to introduce an ArF immersion lithography earlier (Proc. SPIE Vol. 4690 xxix).
In an ArF immersion lithography, a proposal is made to impregnate water between a projector lens and a wafer. A refractive index of water at 193 nm is 1.44, and therefore a pattern formation is possible even if a lens with a numerical aperture (NA) of 1.0 or more is used, and moreover, theoretically NA may be increased to near 1.35. A miniaturization to a level of 45-nm node or lower becomes possible by combination of a lens having NA of 1.2 or more and a super-resolution technology (Proc. SPIE Vol. 5040 p 724).
As a circuit line width becomes narrower, an effect of contrast deterioration due to acid diffusion becomes more serious in a resist composition. This is caused by approaching of a pattern size to an acid diffusion length whereby leading to lowering of mask fidelity, worsening of LWR (Line Width Roughness), and deterioration of pattern rectangularity. Accordingly, to fully enjoy the favor owing to a shift to a shorter wavelength of a light source and to a higher NA, increase of a dissolution contrast or suppression of an acid diffusion is necessary ever than before in the material.
Poly hydroxy styrene (PHS), which has been widely used as a base resin of a KrF resist composition, shows excellent alkaline-dissolution properties, but an insufficient transparency to an ArF beam (193 nm) has been a problem; and thus, for an ArF resist composition, an attempt has been made to change to a material, represented by a (meth)acrylate resin, which has a carboxylic acid as an alkaline-soluble group.
However, because a carboxylic acid is a stronger acid than a phenolic acid of PHS, a resist film swells easily during development, whereby causing poor LWR and pattern fall.
To reduce swelling, lowering of lipophilicity in a base resin is effective; and it has been known that LWR can be improved by lowering lipophilicity by using a unit that protects a soluble carboxylic acid with an acid-labile group having a monocyclic structure. However, when a base resin with reduced lipophilicity is used, there occurs a problem that sufficient rectangularity cannot be obtained in a fine pattern because of an insufficient dissolution contrast.
In order to suppress an acid diffusion, a structure of a photo acid generator (PAG) plays a key role; and this aim has been accomplished to a certain degree by development of a photo acid generator which is stable and yet gives the acid generated therefrom adequate acidity and bulkiness (for example, SF5Ad-03, SF2Ad-03, and SF5Ad-tBu12).
However, in the most advanced lithography, a pattern size approaches to an acid diffusion length; and thus, acid diffusion needs to be suppressed more than ever.
To further suppress an acid diffusion, addition of a quencher component that captures an acid generated by light irradiation is effective; and thus, as the quencher, nitrogen-containing organic basic compounds represented by a primary, a secondary, and a tertiary amines have been widely used. However, these nitrogen-containing organic compounds bring about a size difference between a dark area (large unexposed area) and a bright area (large exposed area) due to being eccentrically located in a resist film and volatilization (chemical flare) from a resist film surface layer, and this in turn leads to difficult surface dissolution and the like that cause a poor profile.
As an illustrative example of other quenchers, a quencher with a type of an onium salt can be mentioned. For example, in Japanese Patent No. 3912767, a resist composition concurrently using a compound that generates an alkanesulfonic acid substituted with a fluorine atom at its α-position and an onium salt of an unfluorinated alkanesulfonic acid, thereby giving a small sparse-dense dependency, especially a small sparse-dense dependency in a line-and-space, has been proposed. Although a detail of this effect is not described, it is supposed that this is caused because the fluorine-containing sulfonic acid generated by photo-exposure reacts with the onium salt of the unfluorinated alkanesulfonic acid for salt-exchange to form the unfluorinated alkanesulfonic acid and an onium salt of the fluorine-containing sulfonic acid whereby exchanging a strong acid (fluorine-containing sulfonic acid) to a weak acid (unfluorinated alkanesulfonic acid). In other words, it is supposed that the onium salt of the unfluorinated alkanesulfonic acid acts as a quencher (acid-deactivator) to a strong acid generated by photo-exposure. A similar proposal to the foregoing is also described in Japanese Patent Laid-Open Publication No. 2009-244859. In this document, an onium salt of an alkanesulfonic acid having a specific structure is proposed; and it is reported that a pattern profile and so on resulted therefrom are excellent.
These weakly acidic onium salt quenchers are generally nonvolatile; and thus, there is no fear of a chemical flare as mentioned above, so that effect to improve rectangularity of a pattern may be expected. Especially in a combination with the foregoing low-lipophilic base resin, rectangularity can be improved while keeping a good LWR level; and thus, a lithographic performance can be enhanced complementarily.
On the other hand, these weakly acidic onium salt quenchers have a lower quenching performance to a strong acid as compared with a nitrogen-containing organic compound such as an amine; and thus, there is a tendency not to adequately suppress acid diffusion so that there is a fear of giving a tapered form and a footing profile in a fine pattern.
In addition, there is a serious problem of pattern fall caused by reduction of an absolute size. Especially when these weakly acidic onium salt quenchers are used, a weak acid generated by the salt-exchange reaction remains within a resist film without being neutralized, so that penetration of an alkaline developer into a pattern becomes easier; and thus, a problem of pattern fall becomes more serious.